usttrace: allow spaces in command-line arguments

The usttrace uses $*, which expands arguments without preserving their
boundaries.  This causes arguments that include spaces to be mangled:

[0] "echo"
[1] "hello world"

becomes:

[0] "echo"
[1] "hello"
[2] "world"

Use "$@" instead so that argument boundaries are preserved.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>